Johnson Electronics sells electrical and electronic components through catalogs. Catalogs are updated and printed every year. The production cost is $$5$ per each catalog. Data indicate that, on average, each printed catalog generates a profit of $$35$ from sales $($i$.$e$.,$ $$40$ revenue$).$ What is the optimal service level for the catalog printing decision?

Please identify Cu $($under$-$stocking cost$)$ and Co $($over$-$stocking cost$)$ to calculate the critical ratio, i$.$e$.,$ the optimal service level. Please briefly explain your calculation logic to show the work.